Polymers and copolymers of C.sub.2 -C.sub.10 olefins, particularly copolymers of ethylene and higher alpha-olefins are produced in gas phase, fluid bed reactors, see for example, Karol, et al. U.S. Pat. No. 4,302,566. In the fluid bed reactor, the monomer or monomers are provided to a fluid bed reactor at pressures slightly above the normal operating pressure of the reactor, which is typically around 200-350 psig. The monomer or monomers are frequently compressed to a pressure above the reaction pressure of the polymerization reactor in order to fed monomer to the polymerization reactor. However in some regions of the world, monomers are delivered to a polymerization facility at very high pressures, sometimes in excess of 1,000 psig. In these instances the monomer feed pressure is typically reduced to a pressure 50-100 psi above reaction pressure through a series of expansions. Reducing the pressure of the monomer feedstream through a series of expansions helps to improve control of the monomer feedstream and also helps to prevent pressure surges within the reactor.
After the monomer is fed into the reactor it reacts with a catalyst to form a polymer. The polymer is then discharged from the reactor. The resulting polymer may also contain gaseous unpolymerized hydrocarbon monomers. An inert gas, typically nitrogen, may also be added to the polymer product stream to aid in the transfer of the polymer product from the reactor and to reduce the risk of explosion if the hydrocarbon monomer content becomes excessive in the presence of oxygen. The polymer product is separated from the gas stream through the use well known methods and devices in the art, see for example U.S. Pat. No. 4,372,758. The gas stream, which still contains the various hydrocarbon monomers, can then be cooled to condense and remove as much of the monomer gases as possible. For economic considerations, it is desirable to recover the monomer gases and recycle the monomer gases to the reactor. Furthermore, environmental regulations require proper disposal of hydrocarbons, such as incineration or other suitable techniques, in order to meet emission standards.
Cooling water is sometimes employed to recover the monomer gases. However, cooling water is ineffective in recovering most of the monomers because the monomer gases condense at temperatures lower than the temperature of the cooling water. Refrigeration equipment is often necessary to chill water, brine or other suitable media used in the condensation and recovery of the hydrocarbon monomer gases. However, refrigeration equipment is expensive to install, expensive to operate and may be a significant operational problem due to extensive maintenance requirements.
Accordingly, a need exists to provide a reliable, inexpensive method for the recovery and recycle of condensable monomer gases from a gas phase fluidized bed reactor. Preferably, the cooling source should be readily available and inexpensive to operate.